The present invention relates to a zoom lens system, and more particularly to a compact zoom lens system consisting of a negative first lens group, a positive second lens group, and a negative third lens group. The lens system has advantages of compact lens composition and good image quality.
Generally, the zoom lens system is classified into two categories. The first category is so-called retrofocus type zoom lens system, which typically consists of a front lens group having negative refractive power and a rear lens group having positive refractive power. A separation is reserved between the front lens group and the rear lens group, and it is possible to vary the separation to vary the focal length of the lens system. The second category of the zoom lens system is telephoto type zoom lens system which typically consists of a front lens group having positive refractive power and a rear lens group having negative refractive power. A separation is also reserved between the front lens group and the rear lens group for zooming operation.
In the well-known compact zoom lens system nowadays, it is intended to shorten the back focal length so as to compact the zoom lens system. It is noted that the zoom lens system used in this field is typically composed of a first lens group with positive refractive power and a second lens group with negative refractive power. In such an arrangement, it is possible to shorten the total length of the lens system.
However, in the prior art, the aperture diaphram of the lens system is mostly designed in the first lens group. During zooming operation in telephoto position mode, the F number (F#) of the lens system will be increased according to the increase of the focal length of the lens system. As a result, the lens system of prior art is only suitable to be used in bright circumstances.
To overcome the problems of the prior art, a three lens groups type zoom lens system was developed in this field. For example, Japanese Laid Open Patent No. Sho 63-25613 disclosed a compact zoom lens system consisting of three lens groups. It is found that the first lens group of the prior art Japanese Patent is fixed, so that the zooming ratio of which is restricted. U.S. Pat. No. 4,983,027 also disclosed a zoom lens system consisting of three lens groups. However, the first lens group of this prior U.S. patent is designed to have a feature of positive refractive power, which results the zoom lens system in worse chromatic quality when operated at wide angle position.